1. Technical Field
The present invention relates to a light-emitting-element driving circuit, and in particular to a light-emitting-element driving circuit having a voltage boosting circuit unit.
2. Background Art
In recent years, various electronic devices such as a portable phone are equipped with a light-emitting element driving circuit. When the light-emitting-element driving circuit has a voltage boosting circuit unit, the voltage boosting circuit unit boosts the voltage so that a terminal voltage of a light-emitting element becomes a predetermined voltage. With this configuration, light is emitted from the light-emitting element with a desired brightness or the like.
As a technique related to the present invention, for example, Patent Literature 1 (JP 2005-11895 A) discloses, as a light-emitting-element driving circuit, an LED driving circuit which drives an LED with a battery, comprising a constant current circuit which is inserted on a side of an anode or on a side of a cathode of the LED and which controls the current flowing in the LED to a predetermined target value, and a resistor connected to the side of the cathode of the LED and downstream of the constant current circuit. The LED driving circuit further comprises a battery in which a voltage changes according to a remaining capacity within a range including a predetermined voltage value when a sum of a forward direction voltage drop of the LED, a drive voltage of the constant current circuit for the predetermined target value, and a voltage on both ends of the resister for the predetermined target value is the predetermined voltage, and a voltage boosting circuit which is connected between the battery and the LED, which boosts and outputs the battery voltage to a voltage greater than or equal to the predetermined voltage when a switch in the voltage boosting circuit is switched ON, and which outputs the battery voltage without any processing when the switch is switched OFF. The LED driving circuit additionally comprises a control circuit which is connected to the constant current circuit, and which detects a relationship in magnitude between the battery voltage and the predetermined voltage and switches the switch of the voltage boosting circuit ON only when the battery voltage is lower than the predetermined voltage.
In the light-emitting-element driving circuit, a voltage on one terminal of the light-emitting element may be input to the voltage boosting circuit unit as a feedback voltage, and the necessary voltage boosting operation may be executed by the voltage boosting circuit unit. However, when there is an abnormality such as an open failure of the path in a feedback loop path for inputting the voltage on the one terminal of the light-emitting element to the voltage boosting circuit unit as the feedback voltage, a suitable feedback voltage cannot be input, and a protection function of the voltage boosting circuit unit may be activated and the voltage boosting operation may be stopped. Because of this, when there are a plurality of light-emitting elements for which the voltage is to be boosted by the voltage booting circuit unit, there is a problem in that light may not be emitted from light-emitting elements other than the light-emitting element provided in the feedback loop path.